The present invention relates to a flat electroluminescent screen or panel. Such a screen makes it possible to display a large quantity of graphic and/or alphanumeric informations and is used as a visual display terminal in portable computers, in telematic terminals, such as the Minitel, or as a television screen.
Conventionally, an electroluminescent screen comprises a substrate on which are stacked layers of electrically conductive materials, electrically insulating layers and a layer of an electroluminescent material, said layers being protected by a counter-plate covering the substrate. Such an electroluminescent screen or panel is more particularly described in the article "Practical application technologies of thin-film electroluminescent panels" by Mikio Takeda et al, published in the Proceedings of the SID, vol. 22-1, 1981, pp 57 to 62. FIG. 1 is a sectional view of an electroluminescent screen according to this article.
The different active layers of the screen are deposited on a transparent glass substrate. Deposition firstly takes place of a conductive layer, e.g. of In.sub.2 O.sub.3, which is then etched to form a network of parallel electrodes 4. This is followed by the successive deposition of a first dielectric layer 6, an electroluminescent layer 8 and a second dielectric layer 10. The dielectric layers are e.g. of Si.sub.3 N.sub.4 and the electroluminescent layer of ZnS:Mn.
The second series of electrodes 14 is then etched in a conductive layer deposited on the second dielectric layer. An anchoring layer 12, e.g. of Al.sub.2 O.sub.3, can be positioned between layer 10 and electrodes 14 in order to facilitate the anchoring or attachment thereof.
Finally, the circuit is protected against mechanical action and moisture by a glass counter-plate 18 fixed to substrate 2 by a sealing band 20, the free space between the deposited layers and the counter-plate 18 being previously filled with a filling material 16, such as a silicone oil.
Each intersection between an electrode 4 and an electrode 14 defines an image element constituted by superimposing the first dielectric layer, the electroluminescent material and the second dielectric layer. The two networks of electrodes 4 and 14 thus define a matrix of electroluminescent elements.
An image element has a certain fragility. Thus, it is not uncommon for an electric breakdown to occur in an image element, which generally brings about a deterioration of at least one of the two control electrodes associated with said element. Thus, the deteriorated electrode portion located beyond the breakdown zone is no longer supplied. The image elements associated with said electrode portion can then no longer be addressed and thus no longer emit light.
Thus, a breakdown in an image element leads to a display fault on a row or column portion of the display. As this fault is not acceptable, methods have been proposed to prevent the deterioration of electrodes when an electric breakdown occurs in an image element.
A first solution is proposed in the article "Thin-film electroluminescent displays produced by atomic layers" by T. Sutela published in the journal Displays, April 1984, pp 73 to 78. This method consists of making short incisions in the electrodes parallel to the direction thereof and level with each image element. The function of these incisions is to stop the propagation of electric breakdowns, in accordance with a principle identical to that of a firebreak in a forest.
This method suffers from the disadvantage of reducing the emissive surface, because the anti-propagation incisions have a non-negligible width of approximately 10 to 20 microns and must be numerous in order to be effective.
A method making it possible to reduce the number of electric breakdowns is also described in GB-A-2096814. This method consists of applying a writing compensation pulse before the refreshing pulse and a refreshing compensation pulse after the refreshing pulse and before the following writing pulse. These compensation pulses have opposite signs to the writing and refreshing pulses and their intensity is sufficiently low not to act on the image elements.
The two known methods described hereinbefore aim at reducing the number of electric breakdowns or the effect of an electric breakdown on an electrode. However, these methods do not provide any solution as soon as an electrode has been damaged. Thus, they do not obviate a display fault on the electrode portion beyond the deteriorated portion.
The object of the invention is to make it possible to continue to control the display of the image elements located beyond the deteriorated portion of the electrode, i.e. to limit the display fault to the single image element destroyed by the electric breakdown.
To achieve this objective, the invention proposes etching counter-electrodes on the inner face of the protective counter-plate and to connect each counter-electrode to the two ends of a control electrode of the electroluminescent screen.
In this way, the control electrodes are supplied by their two ends. Thus, when a breakdown appears on a control electrode, as a result of the electric breakdown of an image element, the electrode portion beyond the breakdown continues to be supplied or energized.
All the image elements, except that where the electric breakdown has appeared are then supplied. Thus, the display fault remains limited to a single image element, which can virtually not be detected by an observer.